In wellbores having mud with high salinities, the electromagnetic couplings between the NMR probe (i.e., sensor) and the conductive mud may deteriorate the performance of the NMR logging tool, resulting in inconsistent signal amplitudes. This is known as the “salinity effect.” For example, the quality factor (“Q”) of the MR Scanner coil decreases by a factor about six as the wellbore conductivity increases from 0 Siemens per meter (“S/m”) to 20 S/m. The NMR logging tools are calibrated using a non-conductive sample. However, if the mud in the wellbore has a high salinity, using the calibration performed with the non-conductive sample underestimates the porosity of the formation. Some NMR logging tools are also calibrated using conductive samples. During logging, electrical measurements are performed that are sensitive to the conductivity. These measurements are then used to correct for the salinity effect. However, when the corrections are large, the resulting uncertainties in the porosity become also large.
Smaller echo spacing may help improve the accuracy of total measured NMR porosity, especially in unconventional reservoirs where transverse relaxation times are short. One factor limiting the echo time reduction is the recovery period following each radio frequency (“RF”) pulse during which the NMR signal cannot be detected. The receiver recovers from the high power RF pulse exponentially, and the time constant t of the recovery is proportional to Q. As such t=2Q/w0, where w0 is the angular operating frequency. At low operating frequencies, like in NMR logging tools, the recovery time may be even longer, making it difficult to reduce the echo spacing and accurately measure the total formation porosity. The recovery time, or ring-down time, is known as the “ringing.” Furthermore, at low operating frequencies and high receiver Q, the NMR signal may be delayed in time and distorted in shape, which complicates the quantitative signal analysis. To decrease the receiver Q, the user may add a damping resistor in parallel with the probe capacitor; however, this also reduces the signal-to-noise ratio (“SNR”).